Conventional recovery facilities for ethylene plants via fractionation generally involve compression of cracked feed gas to high pressures, 400-600 psia, e.g., about 550 psia, followed by extensive chilling and fractionation for products separation. The normal sequence of fractionation is demethanization, followed by deethanization, depropanization and debutanization. For purposes of comparison such process will be termed herein "Base Case". It may be noted that such process requires the entire gaseous effluent of cracking to be compressed to, e.g., 550 psia, of which about one third is tail gas, viz., H.sub.2 +methane. However, tail gas is not required to be compressed to that level--because for subsequent use it is generally let down to much lower pressure--and the energy used up in doing so cannot all be recovered. The present invention aims to avoid high compression of tail gas.
A high pressure process is discussed by J. R. Fair et al. in Chem. Eng. Prog., 54 (No. 12) 39-47, December 1953. Particular emphasis is placed on the separation of the gas into two fractions: one comprising methane and lighter materials and the other comprising ethylene and heavier materials, viz., demethanization which is said to be the major step. The most popular level of feed gas compression in the United States is stated to be 450-600 psia. In this article, feed gas, from which the majority of C.sub.5 and heavier components have been removed, is compressed to this level. The authors note that the principal energy requirements for demethanization are feed gas compression and refrigeration. It is also mentioned that pre-removal of unsaturated C.sub.4 and heavier materials may be desirable in some instances. U.S. Pat. No. 2,938,934 discloses a method wherein, in an initial depropanizer, C.sub.4 and heavier materials are removed as bottoms. The overhead gas is then compressed to 450 psia and passed to a demethanizer; the deethanizer follows the demethanizer. Therefore, high compression of tail gas is not avoided. In U.S. Pat. No. 3,729,944 a cracked gas is subjected to staged compression with interstage cooling and a gas-liquid mixture therefrom is introduced into a first fractionation zone at 177 psia. The C.sub.4+ materials are removed as bottoms and the overhead is compressed to 335 psia and processed for further fractionation/separation. Here also there is compression of tail gas to high levels.
Other literature of general interest includes:
.cndot. Pratt and Foskett, Trans. Amer. Inst. Chem. Engrs. 42, 149 (1946).
.cndot. W. K. Lam et al., Oil and Gas Jl. 111, May 18, 1970.